1. Field of the Invention
The present invention relates to an optical length controllable Fabry-Perot etalon, and more specifically to such an optical filter wherein tuning is achieved by controlling a movable portion made of silicon. The present invention can be made by usual LSI (Large Scale Integration) techniques and features very simple assembly and low manufacturing costs.
2. Description of the Prior Art
In order to attain highly effective communications using optical fibers, it is known to utilize multichannel wavelength division multiplexing arrangements. In such arrangements it is required to select a desired optical wavelength (viz., channel) from a plurality of optical channels. One useful component for achieving this optical channel selection is a Fabry-Perot etalon tunable optical filter.
One known tunable Fabry-Perot etalon (viz., interferometer) is disclosed in an article published in Electronics Letters, Jul. 16, 1987, Vol. 23, No. 15, pages 781-783 entitled "Pigtailed High-Finesse Tunable Fibre Fabry-Perot Interferometers with Large, Medium and Small Free Spectral Ranges" by J. Stone and L. W. Stulz (prior art paper 1).
Stone et al. disclose three kinds of fiber Fabry-Perot (FFP) tunable filters. For all three types of device, the fiber ends are encapsulated in standard glass or ceramic ST ferrules. The fiber ends are polished and coated with multi-layer dielectric mirrors. The ferrules are held in alignment with either a split zirconia sleeve or a solid zirconia sleeve. This assembly is then placed in a cylindrical PZT piezoelectric shell, and the ends of the shell are epoxied (bonded) to the ferrules.
Stone et al. (prior art paper 1) uses a cylindrical piezoelectric shell and also utilizes a zirconia sleeve for preventing fibers from being undesirably displaced. This prior art, however, has encountered problems that an expensive component such as piezoelectric shell must be used and hence leads to high manufacturing costs. Further, a large number of elements are employed with the result of complex assembly. Still further, this prior art is found unsuitable for mass production using large scale integration techniques.
Another prior art of tunable Fabry-Perot etalon is disclosed in an article published in Journal of Lightwave Technology, Vol. 7, No. 4, April 1989, pages 615-624 entitled "Angled-Tuned Etalon Filters for Optical Channel Selection in High Density Wavelength Division Multiplexed System" by Anatoly Frenkel and Chinlon Lin (prior art paper 2).
Frenkel et al. discloses an in-line fiber-coupled angle-tuned Fabry-Perot etalon. Lensed collimated-beam type single-mode fiber connectors are used for collimating and refocusing the beam into the fiber. A Fabry-Perot etalon is mounted on a rotating stage which accomplishes the wavelength selection simply by angle adjustment.
This mechanical type of angle-tuned or rotatable Fabry-Perot etalon is advantageous for lowering an overall manufacturing costs of the system. However, as the thickness of the etalon increases, displacement of beams which issue from the etalon becomes larger and accordingly increases an insertion loss.